I've had eggs pipping when I went into lockdown, so I've had eggs hatch at 19 days.

If the average incubating temperature is a little high, eggs will hatch early. If the average incubating temperature is a little cool, they can hatch late. Heredity can affect whether they hatch a little early or late. I'm not sure about this one, but I've heard that smaller eggs will hatch a little earlier than larger eggs from the same breed.

I just had my first egg hatch at Day 19. I know my day count was correct because I marked on the calendar the day she set on the egg. It freaked me out until I started searching and reading about hatching dates.

I hope my second egg (set about a week after the first egg) hatches at Day 19 so Mama doesn't have to keep sitting on it. =O